Simple definitions in dictionaries — an example

In a good English dictionary, the definitions are simple, but still useful. Look at the difference between a complicated definition and a simple explanation of the word ear:

ear = the organ of hearing and equilibrium in vertebrates, in mammals consisting of an external ear (...), a middle ear (...), and a liquid-filled inner ear, with sensory nerve endings for hearing and balance. [Random House Webster's Electronic Dictionary]

Everybody knows what ears are. Why is this definition so complicated? It could look like this:

Your ears are the two parts of your body, one on each side of your head, with which you hear sounds. [Collins COBUILD English Dictionary]

This definition uses simpler words than the first one. It doesn't use difficult, scientific words like equilibrium and vertebrates. This definition is a real English sentence — it sounds like a friend who is explaining a word to you. It is very easy to understand.